Method of rounding wire end



Aug. 14, 1956 A. J. LINCOLN METHOD OF ROUNDING WIRE END Filed July 7, 1951 INVENTOR.

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ARA/0L 0 14 T ToAWgX METHOD OF ROUNDENG WINE END Application July 7, 1951, Serial No. 235,637

4 Claims. (Cl. 219-15) The invention is directed to the art of metal working and is more specifically concerned with a method of rounding the ends of wires which are to be used as electrodes in various operations.

When a wire is cut, the section cut usually consists of sharp and irregular edges which, in certain industrial processes are not desirable. For example, in a mercury switch or other thermally expansible metal switch wherein there are provided one or more control electrodes which are contacted by the expansible metal to close a circuit, it is highly desirable in order to avoid arcing, premature contact and pitting of the electrode, that each electrode present a smoothly rounded surface. it is furthermore necessary, in order to assure accuracy of operation of the switch device that each electrode have a uniform size of rounding.

It has been conventional to round such ends of wires by playing a gas flame thereon. However, the use of the gas flame in rounding operations has certain limitations. The degree of roundness is difficult to control and is entirely dependent upon the skill of the operator. Thus, where uniformity in the roundness of the ends of wires of varying diameters of any shape is desired the use of a gas flame is not satisfactory.

Furthermore, for operation upon fine wires of the order of .005 to .050 it has been found that it is difiicult to obtain a small enough gas flame, and it would also be difficult to operate in a vacuum or under a pressurized atmosphere to perform the rounding operation.

It is an object of the present invention to provide a method of rounding the ends of wires of varying diameters wherein the wire or rod need not be confined to round cross-sectional shape but may be square or irregularly shaped. It is a further object of my invention to provide a method of rounding wires wherein a uniformity of roundness from wire to wire is assured. It is a still further object of my invention to provide a method of rounding wires which is readily adaptable to fine or small dimensioned wires.

Other objects and advantages of the invention will appear more readily from the following description and as particularly pointed out in the appended claims, reference being had to the accompanying drawing forming part of this specification, and in which is set out a typical schematic wiring diagram of a circuit used to perform my method.

When heat is applied to the end of a wire the metal becomes soft and flows, but due to surface tension the end tends to assume a substantially spherical shape. However, the consistency in duplicating the spherical or round shape of the end of each wire specimen is dependent upon the control of the heat applied to the wire end. According to my method I use an electrical are as a means of heating the wire ends and thereby eliminate the difliculties encountered in the use of a gas flame.

Any source of D. C. power would be suitable for producing an are for heating the ends of the wires that nited States Patent 2,759,088 Patented Aug. 14, 1956 are to be rounded. provided that suflicient amperage and voltage are available. However, the use of a D. C. are alone would not be satisfactory, since an arc of this type has to be struck (i. e., the electrodes must be manually moved into contact and then separated in order to start the arc) each time a new wire is to be worked on. As a result it would be difiicult to maintain the gap spacing between the electrodes. I propose the use of a high voltage spark to strike the are together with a source of D. C. power supply for the arc.

According to the drawing the basic circuit consists of two portions. Portion 1 serves to generate a high voltage spark and can be termed the igniter portion while portion 2 supplies D. C. power to the electrodes and can be termed a power portion to complete the discharge circuit.

The input into the discharge circuit may be a 60 cycle A. C. In the power portion 2 of the circuit, a magnetic core transformer 3 is directly connected to the power input. In the output conductor 10 of the transformer 3 two rectifiers 4 and 5 are connected in parallel, as in conductors 12 and 13, to accomplish a full wave rectification. In series in the conductor 11 an iron core inductance is placed to suppress the A. C. current and allow only D. C. current to flow to the condenser 7. Placed in parallel across the conductors 10, 11 is a variable resistance 6 which may be a bank of graduated resistances which can be independently thrown into the circuit to gradually allow a buildup of energy in the condenser 7. The capacitor 7 may be a large bank of capacitors of high value which are placed in parallel on the output side of the power portion 2 of the discharge circuit. The variable resistance and inductance serve to control the total power and the duration of the discharge. The portion 2 thus serves to charge capacitor 7 with a D. C. voltage of a definite value before each gap breakdown.

The igniter portion 1 of the discharge circuit contains a magnetic core transformer 19 which is directly connected to the power input and which delivers a high voltage A. C. to the condenser 16 which is connected to the output conductors 17, 18 of the transformer 1%.

The electrode holders 24 and 25 are connected across both portions 1 and 2 of the circuit and the wire specimens are mounted therein. The wire sample to be rounded is mounted in the holders 24 and 25 and must be of the same material in order to produce an arc.

When the electrical power is turned on, a high A. C. voltage (as high as 10,000 volts) jumps across the gap between 24 and 25 which permits the D. C. are to start. A circuit according to the one above described may use 300 volts D. C. and 0.1 to 15 amperes.

In order to ensure a duplication of roundness from one sample of wire to another, certain conditions of the circuit must be maintained. The distance between the electrodes must be between 2 to 4 diameters of the wire sample. The D. C. voltage input to the electrodes should be maintained constant at about 4 the A. C. voltage input to the circuit. For working on fine wire samples it has been found that the D. C. current supplied to the electrodes should be between the limits of 0.1 to 15 amperes. The time during which the wire sample should be subjected to the D. C. are should not exceed 3 seconds for fine wire and should preferably be maintained at 1 second. However, all these variables except the gap spacing may be changed according to the degree of roundness desired and the size of wire being worked upon. Furthermore, several changes may be made in the suggested circuit without adversely affecting the end product. For example, instead of rectifying tubes 4, 5 or copper oxide rectifiers a motor-generator set may be used.

Samples made according to my method applying a 300.

volt D. C. are to the electrodes spaced at a distance of 3 mm. have produced uniform roundness in wires of small cross-sectional dimensions. For example, a .005 round wire when subjected to a current of .25 amperes under the above conditions had a rounded end of .016" uniformly formed thereon; and a .045" square wire when subjected to a current of 1.5 amperes under the above conditions had a rounded end of .063 uniformly formed thereon. In all cases, the size of the rounded end is greater than the cross-sectional dimension of the wire.

By virtue of the method disclosed herein the size of roundness of wire ends may be accurately controlled and uniformly reproduced.

Although I have illustrated and described a basic circuit to practice the method of my invention, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims.

Having now described my invention what 1 claim as new and desire to secure by Letters Patent, is:

1. The method of preparing wire electrodes by producing uniformly reproducible rounded ends on fine metal wires of a cross-sectional size between .005" and .050" wherein the size of roundness produced is greater than the cross-sectional diameter of the wire comprising mounting a pair of wire samples in electrode holders, connecting a high voltage spark circuit to said electrodes, connecting a D. C. power supply circuit to said holders, spacing said electrode wire samples a distance of 2-4 wire diameters and subjecting said wire samples to said are for a period of 1-3 seconds.

2. The method of claim 1 wherein the wire samples are subjected to the step of passing a D. C. current of 0.1 to amperes therethrough at the time that the D. C. are is ignited.

3. The method of claim 1 wherein the D. C. power supply circuit is supplied with A. C. voltage and inaud- 4 ing the step of rectifying said A. C. voltage by the means of a motor generator set and supplying D. C. voltage to the electrodes at a constant value of the A. C. voltage input.

4. A method of preparing wire electrodes by producing uniformly reproducible rounded ends on fine metal wires of a cross-sectional size between .005 and .050 and of any cross-sectional shape, wherein the size of roundness produced is greater than the cross-sectional diameter of the wire, comprising mounting a pair of said metal wire samples in electrode holders, spacing said electrodes a distance of 2-4 wire diameters, connecting a high voltage spark circuit consisting of a capacitor supplied with high A. C. voltage by a magnetic core transformer, said electrodes being connected to said capacitor, further connecting said electrodes to A. C. voltage supplied D. C. power transducer including variable resistances and variable capacitors, igniting said arc by actuating said spark circuit, adjusting the variable resistances and capacitors of said transducer to deliver a D. C. voltage to said electrodes at the value of the A. C. voltage input, maintaining said arc discharge for a period of 1-3 seconds, and supplying said voltage at a D. C. current of 0.1 to 15 amperes at the moment the arc is ignited.

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